Electronics Components: What are Operational Amplifiers?

An op amp is a super-sensitive electronic amplifier circuit that’s designed to amplify the difference of two input voltages. Thus, an op amp has two inputs and one output. The output voltage is often tens or even hundreds of thousands of times greater than the difference in the input voltages.

Internally, the simplest op amps consist of several dozen transistors, and more complicated varieties have many more. But you can ignore the internal circuitry and just treat it as a handy device you can use without understanding how it works.

The following describes the function of each of the connections shown in the schematic symbol:

+V and –V: The power supply is provided via two pins usually labeled +V and –V. (These pins may be labeled Vs+ and Vs– instead.) Most op amps require both a positive and a negative voltage power supply, with voltages usually ranging from 6 V to 18 V.

This type of power supply is called a split supply. The indicates that both positive voltage and negative voltage are required. 6 V, for example, means that both +6 V and –6 V are required.

You can build a split supply by using two batteries connected end to end. Here, two 9 V batteries are connected to create a 9 V supply. Note that the +9 V and –9 V are measured relative to ground, which is accessed between the two batteries

Some op amps don’t require split-voltage power supplies. Op amps that use single power supplies have a ground terminal instead of a –V terminal.

Vout: The output of the op amp is taken from the Vout terminal. The voltage at the output terminal can be positive or negative, depending on the voltage difference between the two input terminals. The maximum voltage is usually a few volts less than the supply voltage at the +V and –V terminals. Thus, if the power supply for an op amp is 9 V, the maximum output will be around 7 V or 8 V.

Most op amps can handle only a small amount of current through the output terminal — usually, in the neighborhood of 25 mA or less. The output is passed through an external resistance, designated RL. The other end of this resistance is connected to ground. Thus, the output current that flows through the op amp must eventually end up at ground.

V+ and V–: The two inputs are the V+ and V– terminals. These terminals are sometimes identified by + and – signs inside the triangle. The inputs are called differential inputs because the output voltage, which appears on the Vout terminal, depends on the difference between the voltage of the + and – terminals.

For most op amps, the maximum allowable input voltage is a bit less than the maximum power supply voltage. 12 V is a typical limit. Remember, though, that the difference between the two input voltages is what an op amp amplifies. In many cases, the two input voltages are very close, so the difference is very small.

The polarity of the op-amp output depends on the polarity of the difference between the V+ and V– inputs. Thus, if V+ is greater than V–, the output will be a positive voltage, but if V+ is less than V–, the output will be a negative voltage.

In many op-amp circuits, one input is connected to ground. If the V+ input is grounded, the output polarity is always the opposite of the polarity of the input voltage on the V–terminal.

In other words, negative voltage on V–will give positive voltage on Vout, and positive voltage on V–will give negative voltage on Vout. For this reason, the V–input is often called the inverting input because its polarity is inverted in the output.

If, on the other hand, the V– input is connected to ground, the polarity of the output is the same as the polarity of the input voltage applied to V+. Thus, if V+ is positive, Vout will be positive; if V+ is negative, Vout will be negative. For this reason, the V+ input is called the noninverting input because its polarity is the same in the output — that is, the V–input voltage is not inverted.